1. Field of the Invention
The present invention relates to a zoom lens and an optical instrument using the zoom lens, and in particular relates to a wide-angle zoom lens and an optical instrument using the wide-angle zoom lens suitable for still cameras, such as single lens reflex cameras, electronic still cameras, and video cameras, which are compact and high-efficiency especially preferably correcting lateral chromatic aberration.
2. Description of the Related Art
An interchangeable lens for a single lens reflex camera requires a long back focus in some degree in order to ensure a space for arranging a quick-return mirror and so forth in the image plane side of a lens system. In a digital camera using a photoelectric conversion element such as a CCD, a long back focus is also required in order to ensure a space for arranging a low-pass filter, an infrared cutting filter, and so forth.
Hitherto, as a wide-angle zoom lens with a focal length at a wide-angle end of less than 35 mm on a zoom lens basis for a 35 mm single lens reflex camera, a so-called negative-lead type short zoom lens having a negative lens group and a positive lens group led by a negative refractive power is disclosed in Japanese Patent Laid-Open No. 5-88084 (corresponding to U.S. Pat. No. 5,452,134) and Japanese Patent Laid-Open No. 7-234360, for example.
This is a retrofocus type lens system which is a zoom type system capable of ensuring a long back focus with the most simple lens structure. This zoom type is advantageous for cost cutting due to the simple structure.
Hitherto, in the wide-angle zoom lens described above, excellent correction of image plane curvature and lateral chromatic aberration has been a particular challenge for obtaining good optical performance. The problem occurs due to the asymmetric arrangement of refractive powers in which the object side of a lens system has a negative refractive power and the image side has a positive refractive power across a diaphragm therebetween, and especially with regard to lateral chromatic aberration, a large amount of aberration remains in a range from the central part of the image toward the peripheral part, and a color halo may be thereby produced especially when taking a picture of an object with high-contrast brightness. One method for correcting the lateral chromatic aberration involves the use of anomalous dispersion glass; however, it may have a tendency to increase the number of lenses in the lens system.
As for chromatic aberration, other than a correction method in which glass materials having different dispersion are combined together, there are corrected optical systems in which a lens surface or a part of the optical system is provided with a diffractive optical element having a diffraction function, which are disclosed in Japanese Patent Laid-Open No. 4-213421 (corresponding to U.S. Pat. No. 5,044,706), Japanese Patent Laid-Open No. 6-324262 (corresponding to U.S. Pat. No. 5,790,321), Japanese Patent Laid-Open No. 9-197274 (corresponding to U.S. Pat. No. 6,067,196), Japanese Patent Laid-Open No. 9-211329 (corresponding to U.S. Pat. No. 5,872,658), and U.S. Patent No. 5,268,790, for example.
In general, in a negative lead type zoom lens comprising two lens groups of a first group having a negative refractive power and a second group having a positive refractive power, widening the picture angle is comparatively easy and a predetermined back focus is readily obtained.
However, in order to obtain excellent optical performance over the entire optical power range while widening the picture angle, it is necessary to properly set the arrangement of the refractive power and the lens configuration of each lens group. When the arrangement of the refractive power and the lens configuration of each lens group are improper, changes in chromatic aberration accompanied by a variable power become larger even when using a diffractive optical element, so that excellent optical performance over the entire optical power range is difficult to be obtained.
It is an object of the present invention to provide a zoom lens that is a negative-lead type zoom lens led by a lens unit having a negative optical power, in which various kinds of aberration including chromatic aberration are excellently corrected by properly setting the lens configuration of each lens unit so as to achieve high optical performance over the entire range of a variable power while the entire lens system is miniaturized and designed to have a wide angle.
In particular, the invention is directed to a compact zoom lens with a wide angle including a wide-angle region which has a focal length of substantially 28 mm on a 35 mm single lens reflex camera basis, and having excellent optical performance especially including favorably corrected lateral chromatic aberration.
In order to achieve the object mentioned above, according to a first aspect of the present invention, a zoom lens comprises a first lens unit having an overall negative power and a second lens unit disposed behind the first lens unit, having a positive power, and during zooming, at least the second lens unit is moved so that the space between the first lens unit and the second lens unit is changed, in which
(A1) the first lens unit comprises at least two negative lens elements and at least one positive lens element,
(A2) the second lens unit comprises at least two positive lens elements and at least one negative lens element,
(A3) at least one of the first lens unit and the second lens unit comprises a diffractive optical element, and
(A4) the following conditions are satisfied:
DaW greater than DaT 
and
10 less than |fDOE|/ft less than 500, 
where DaW and DaT represent the spaces between the first lens unit and the second lens unit at a wide-angle end and at a telephoto end, respectively;
fDOE represents the focal length only due to diffraction of the diffractive optical element; and ft represents the focal length of the entire system at the telephoto end.
In accordance with a second aspect of the present invention, a zoom lens comprises a first lens unit having a negative optical power, a second lens unit having a negative optical power, and a third lens unit having a positive optical power, from an object side in that order, in which during zooming, the spaces between the first lens unit and the second lens unit and between the second lens unit and the third lens unit are changed, and at least one of the first lens unit, the second lens unit, and the third lens unit comprises a diffractive optical element.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.